Can someone please explain to me how non-random winnowing or "decimation" occurs within a population? Everything I read only considers favorable mutations in isolation, ignoring the obvious fact that favorable mutations inevitably arise in the same animals as unfavorable mutations. And that in any case, the fact of any given allele having a decisive impact in reproductive survival is pretty slim. The "longer legs mean a better chance of outrunning predators" explanation only goes so far...can somebody help?